how to deal with side IO operations in recursive functions in Haskell?

Question

I am a complete newbie in Haskell. I am wondering why this pice of code:

main :: IO ()
main = run theWorld presentationIO

run dom showDom = do
       event <- readCommand
       dom' <- updateDomain dom event 
       showDom dom'
       run dom' showDom

.. does not work and how to solve it. The error is :

simpleExampleTextGameV.0.2.hs:96:16: error:
    • Couldn't match expected type ‘IO World’ with actual type ‘World’
    • In a stmt of a 'do' block: dom' <- updateDomain dom event
      In the expression:
        do event <- readCommand
           dom' <- updateDomain dom event
           showDom dom'
           run dom' showDom
      In an equation for ‘run’:
          run dom showDom
            = do event <- readCommand
                 dom' <- updateDomain dom event
                 showDom dom'
                 ....
   |
96 |        dom' <- updateDomain dom event 
   |                ^^^^^^^^^^^^^^^^^^^^^^

In order to reproduce it, you could launch the rest of code: https://github.com/agutie58/landOfLispInHaskell/blob/main/simpleExampleTextGameV.0.2.hs

Thanks in advance!

Fyodor Soikin · Accepted Answer · 2021-07-18 15:45:20Z

8

The function updateDomain, as defined on line 66, returns World.

But the "left arrow" on line 96 requires a monadic value on the right of the arrow, in the current monad, which in your case is IO.

So this means that updateDomain should return IO World, not just World, if you want to use it on the right side of the left arrow in a do block.

The right solution, however, is not to make updateDomain return IO World, but rather to drop the left arrow. Instead, use let to bind dom':

run dom showDom = do
  event <- readCommand
  let dom' = updateDomain dom event 
  showDom dom'
  run dom' showDom

answered Jul 18, 2021 at 15:45

Fyodor Soikin

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6 Comments

chepner Over a year ago

Or dom' <- updateDomain dom <$> readCommand.

Mandor Over a year ago

Thank you for all your comments Fyodor and also chepner, you are absolute all right, it works perfectly after that. But I do no fully understand the solution. It means that if a certain function like run that returns IO b (in this case run :: World -> (World -> IO a) -> IO b ), it means that all actions inside should be IO something ? I assume that event <- readCommand executes this action, whereas let is different (binding function).

Mandor Over a year ago

And let say that in another case, i was forced that a function foo had to return something as IO World. How can I force it !? something like: foo world = (putStr "") world !? It is little strange no!?

Mandor Over a year ago

In pure functions the chain of types it is clear, but when it comes IO actions it turns little confusing to me (and more if they are in "do" statement).

Fyodor Soikin Over a year ago

I cannot answer more questions in the comments. If you have more questions, please post them as separate questions.

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